1. Field of the Invention
The present invention relates to an information storage disk for recording, reproducing, and erasing information by irradiation of laser beam.
2. Description of Prior Art
Since the disclosure by S. R. Ovsinsky, et al. in U.S. Pat. No. 3,530,441 that the thin films of Te.sub.85 EiGe.sub.15, Te.sub.81,Ge.sub.15 S.sub.2 Sb.sub.2, etc. show reversible phase change by irradiation of laser beam and the like, there has been conducted development of an information storage disk using a chalcogenide alloy thin film as a recording layer by which recording, reproducing and erasing of information can be performed.
As one of the targets for development of the information storage disk of this type, there is a task to reduce the energy necessary for recording and erasing, i.e., to enhance sensitivity. British Patent Publication 2,079,031 specification discloses a tri-layer structure optical information recording medium made by inserting a spacer layer between the recording layer comprising a chalcogenide alloy thin film and a reflective layer comprising a metal thin film having large optical reflectivity such as Au, Al, etc. This is a means of enhancing optical absorptivity by utilizing the interference effect so as to obtain enhanced sensitivity. Further, Japanese Patent Publication (Unexamined) No. 63-9040/1988 specification discloses an optical information recording medium having the above-mentioned tri-layer structure, of which the reflective layer comprises Ni--Cr alloy thin film. This schemes, by using Ni--Cr alloy having small thermal diffusivity as a reflective layer, to reduce thermal diffusion to the surrounding at the laser beam irradiated part in recording and erasing and, by elevating the temperature of the recording layer in good efficiency, to enhance sensitivity.
However, there has been a problem that, when a Ni--Cr alloy is used as a reflective layer, the disk becomes weak to thermal shock. In the information storage disk for performing recording and erasing by phase change, in general, recording is performed by partial amorphization of recording layer, and erasing is performed by crystallization. This is because the amorphization is more suitable for forming a minute mark. However, as the recording layer has an amorphous phase in forming, it requires to be converted into a crystalline phase before use. This treatment is called an initialization. In order to effect good initialization in a short time, it is desirable to effect simultaneous initialization over a wide range by irradiating with a laser beam having a large spot diameter. However, when an information storage disk using Ni--Cr alloy as a reflective layer was initialized while rotating with an Ar laser having a spot diameter of about 40 .mu.m and subjected to thermal shocks of charging in and taking out of a room with constant temperature kept at 90.degree. C., cracks were generated on the reflective layer. It was due to the stress caused by the initialization and the expansion and shrinkage of each layer at the time of the exertion of the thermal shock, and is presumed to be attributed to the small malleability of the reflective layer.